Of these types of clamp device, a clamp device has conventionally been described in Patent Literature 1 (Japanese Patent No. 4297511). The conventional technology is configured as follows:
A clamping rod and a piston are arranged one above the other in a housing, and a plurality of engagement members (grippers) are circumferentially arranged on an outer circumference of an upper part of the clamping rod. The engagement members are retained at a raised position by upward hydraulic pressure exerted on an annular pressure-receiving member.
During lock driving, a workpiece is first caused to move downward with respect to the plurality of engagement members. This causes a hole in the workpiece to be fitted on the engagement members. After that, the piston causes the clamping rod to move downward. Then, a wedge face provided in the upper part of the clamping rod causes the engagement members retained at the raised position by the annular pressure-receiving member to move radially outward, so that thin-walled grip parts provided in upper parts of the engagement members strongly grip an inner circumferential surface of the hole in the workpiece. Next, in the gripped state, the piston causes the engagement members and the annular pressure-receiving member via the wedge face of the clamping rod to move downward in an integrated manner (lock stroke), and the engagement members pull the workpiece downward so that the workpiece is fixed to a stationary-side member. In the lock state, the piston has a lock margin stroke formed below the lock stroke.
The conventional technology thus configured is such that in a case where, during downward movement of the clamping rod for locking, the grip parts of the engagement members slip downward with respect to the inner circumferential surface of the hole in the workpiece, the clamping rod and the piston, which have been wedge-engaged on the engagement members, move downward to the locking margin stroke region beyond the lock stroke region, so that the engagement members and the annular pressure-receiving member, too, move downward in an integrated manner. This causes a collar portion provided in an upper part of the annular pressure-receiving member to push a valve member open downward, and at the same time, the collar portion is received by a stopper portion provided in the housing (see FIGS. 5 and 6 of Patent Literature 1).
Since opening of the aforementioned valve member causes a decrease in pressure of pressurized air for use in detection, a malfunction due to the slip is detected by detecting the decrease in pressure with a pressure switch.
That is, in the conventional technology, the annular pressure-receiving member, which moves downward in an integrated manner with the engagement members (grippers), serves as an actuating portion for a malfunction.